[FEATURE] JavaScript-based icon API

A new JavaScript-based icon API is introduced with this patch.
As first step, some spinner icons used in the topbar items are
replaced with the icon API.

Resolves: #70583
Releases: master
Change-Id: I24c0649df5ddce2eb8f3191137a01fa7db29209a
Reviewed-on: http://review.typo3.org/43961


Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>